Tubular electrical heating elements are used in a wide variety of heating devices/equipment and provide the means for producing the heat energy required in these devices. Tubular heating elements are generally comprised of an outer tubular metal sheath of a desired diameter and thickness, dependent on the application. Encased within the outer tubular sheath is an electrically conductive resistance wire surrounded by magnesium oxide which acts as an electrical insulator between the wire and the outer tubular sheath. When current flows through the wire, the wire temperature increases and subsequently the temperature of the outer sheath of the element also increases thereby producing heat energy. The ends of such tubular elements are normally mounted in connector bushings which, in turn, are contained in an electrical box which has an electrical supply. The connector bushing normally exits the box and is secured to it by a suitable fastener with the wire having a terminal end adapted for connection to an electrical supply.
A commonly used method of attaching bushings to electrical heating elements uses brazing or welding. Depending on the attachment method used, additional steps must be taken to ensure that the joint between the bushing inner surface and the tubular element outside surface is waterproof and gas tight. If the bushing is brazed, the brazing metal used will form a water and gas tight seal. Other methods involve the use of epoxy or other sealants to ensure a water and gas tight seal. For tubular elements installed in equipment for use in hazardous locations where the possibility of an explosion exits, the interface between the bushing and the sheath of the tubular element must provide a zero tolerance and have sufficient mechanical strength to sustain an explosion. The certified and accepted method currently employed in attaching a terminal bushing to a tubular heating element for use in hazardous locations is brazing. Brazing still employs manual steps although a number of bushings are now brazed by automated brazing machines. In order to ensure the integrity of the joint the currently approved and accepted method of testing the joint is hydrostatic testing. Currently, applicants are not aware of any non-brazed bushings available on the market that have been approved for use in hazardous locations due to the very tight and strong joint required between the bushing and the element sheath. Epoxy, silicone or other means of sealing the joint are not allowed in the construction of heater elements for hazardous applications.
Accordingly, objectives of the present invention are to provide (a) an improved method of attaching a connector bushing to a tubular heating element usable in heating devices intended for use in hazardous locations and (b) an improved tubular element/connector bushing combination as produced by such method. The method provides sufficient mechanical integrity and a zero tolerance fit between the bushing and the element sheath which is strong enough to sustain an explosion while providing a non-flame path in the construction of the heating device.
It is yet a further object of the present invention to provide a means of attaching a connector bushing to a tubular heating element such that the element to bushing joint will have negligible thermal limitations over the entire operating range of the tubular heating element.
It is still a further object of the present invention to provide a means of attaching a connector bushing to a tubular electrical element in a fashion such that routine hydrostatic testing of the joint is not required to satisfy the appropriate safety and standards authorities.